Polyvinylchloride ("PVC"), tetrafluoroethylene-hexafluoropropylene copolymers ("FEP"), polyvinylidinedifluoride ("PVDF"), and other halogen containing polymers are flame retardant materials for the insulation and jacketing of electrical, fiber optic, and composite cables.
Growing concern regarding the toxicity and corrosivity of combustion byproducts from these materials has led to the development of several generations of nonhalogenated flame retardant materials. Many of these products are based upon low cost olefinic polymers, such as polyethylene or polyethylene copolymers. The flammability of these low cost polymers is reduced by adding large amounts (up to 65% by weight) of hydrated mineral fillers such as aluminum trihydrate ("ATH") or magnesium hydroxide (Mg(OH).sub.2). Numerous other mineral fillers are also used alone or in combination with the above fillers for cost dilution or performance modification. Polyethylene may be a poor polymeric host for hydrated mineral fillers due to the high polarity of these fillers in comparison to the nonpolarity of polyethylene. Addition of large amounts of hydrated mineral filler results in compositions having poor physical properties. Moreover, these compositions perform poorly as flame retardant cable jacket compounds, because they tend to drip during burning and fail to form a strong char layer. Formation of a strong, protective char layer is desirable to prevent the typically more flammable core materials from being exposed to the fire.
Developments have also emerged based upon polyethylene copolymers such as polyethylene-vinyl acetate ("EVA"), polyethylene-methyl acrylate ("EMA"), polyethylene-ethyl acrylate ("EEA"), polyethylene-n-butyl acrylate ("EnBA"), and others. These materials have a degree of polarity which improves their acceptance of hydrated mineral fillers resulting in improved filler dispersion, improved physical properties, and improved flame retardancy. However, these materials still suffer in performance due to their poor ability to form a high integrity char during combustion. For example, the decomposition chemistry of EVA is such that a high degree of carbonaceous char forms only after considerable molecular weight/viscosity breakdown occurs. In industry standard vertical cable burn tests, such as IEEE 383, this presents a problem in some cable constructions, because the flame retarded jacket material may drip off prior to the formation of a strong char. The decomposition chemistry of the acrylate based copolymers is different from that of EVA In particular, the viscosity breakdown of these compounds is slower, resulting in a reduced tendency to drip during burning. Unfortunately, these materials produce very little carbonaceous char; instead, decomposition produces a white ash residue, which has very little strength.
Developments aimed at improving the performance of EVA based compounds have been numerous. These developments have often been based upon the addition of a silicone rubber or a high viscosity silicone fluid or upon some type of silane modification of the base resin or filler. Other approaches add thixotropic fillers or viscosity modifiers to prevent dripping during burning. Most or all of these approaches, however, still call for the use of high filler loadings (ca. 60-65%). However, high filler loadings undesirably impose extrusion limitations, reduce elongation, increase density, and reduce impact strength at subzero temperatures.
Improvements have also been made upon compositions based on polyethylene-acrylate copolymers. One of the best performing materials includes a blend of polyethylene and EEA copolymer with less than 50% magnesium hydroxide and 2-5% red phosphorous. Red phosphorous alters the decomposition chemistry of EEA, acting as a char catalyst to increase the amount of carbonaceous char produced during burning. Adequate burn performance can be achieved at these reduced filler levels so that good elongation and good low temperature impact strength are retained at lower compound density. This material is also easier to extrude than competing EVA compounds. This balance of physical properties and fire protection makes the red phosphorous containing material one of the best non-olefinic flame retardant compositions for cable and wire insulation and jacketing. Unfortunately, because red phosphorous liberates toxic phosphine gas during storage, handling, and compounding, the use of red phosphorus is disfavored among many compounders.
In light of the foregoing, there exists a need for an improved flame retardant cable jacketing compound that is capable of meeting marketplace requirements for physical properties, burn performance, and extrudability without raising undue health concerns regarding toxicity. The present invention is directed to meeting this need.